In character Ainz would retreat and not bother fighting.

Assuming he does then his tactics against groups from what we know is to teleport to someone hard to locate then use Perfect Unknowable to take out healers and support fighters. I don't think there's anything that the group could do to stop that. His spell list includes time stop, controlled black holes, reality tears and instant death spells that have varying degrees of strength. The tactic of fighting solo isn't even something that can be counted on as he's also just as likely to use summons or world items if he isn't trying to keep up appearances or preserve resources. From what I know about the other groups the only actual chance they have of hurting Ainz is with turn undead but thats almost definitely not going to one shot him and Ainz will be able to tell the Aqua is a healer/support and will instantly target her with attacks that will kill or at the very least restrain her.

Momon isn't that strong but he still has all of Ainz stats so at the very least there's not much that can hurt him, he can comfortably tank spells like meteor swarm at the very least so Megumin won't one shot him and Aqua might not be able to tell he's undead right away so that damage from her probably wont come into play. Either way Ainz makes a point as Momon to somewhat fight fair and probably wont resort to using his Overlord abilities so he should eventually win but slowly, I'm not sure if he can put down Darkness though, she's taken a hell of a beating before. He might also struggle with anyone with flight but his resistances and inventory of heals should make it a tie really.